DOC PREVIEW
UVA MSE 2090 - Dislocations and Strengthening Mechanisms

This preview shows page 1-2-3 out of 9 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 9 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

11MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningDislocations and Strengthening MechanismsWhat is happening in material during plastic deformation?Chapter Outline Dislocations and Plastic Deformation9 Motion of dislocations in response to stress9 Slip Systems9 Plastic deformation in  single crystals polycrystalline materials Strengthening mechanisms9 Grain Size Reduction9 Solid Solution Strengthening9 Strain Hardening Recovery, Recrystallization, and Grain GrowthNot tested: 7.7 Deformation by twinning,Direction and plane nomenclature in §7.4.2MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningWhy metals could be plastically deformed? Why the plastic deformation properties could be changed to a very large degree by forging without changing the chemical composition? Why plastic deformation occurs at stresses that are much smaller than the theoretical strength of perfect crystals? IntroductionThese questions can be answered based on the idea proposed in 1934 by Taylor, Orowan and Polyani: Plastic deformation is due to the motion of a large number of dislocations.Plastic deformation – the force to break all bonds in the slip plane is much higher than the force needed to cause the deformation. Why? 3MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningIf the top half of the crystal is slipping one plane at a time then only a small fraction of the bonds are broken at any given time and this would require a much smaller force. The propagation of one dislocation across the plane causes the top half of the crystal to move (to slip) with respect to the bottom half but we do not have to break all the bonds across the middle plane simultaneously (which would require a very large force).The slip plane – the crystallographic plane of dislocation motion.Dislocations allow deformation at much lower stress than in a perfect crystal4MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningDirection of the dislocation motionFor mixed dislocations, direction of motion is in between parallel and perpendicular to the applied shear stressEdge dislocation line moves parallel to applied stressScrew dislocation line moves perpendicular to applied stress25MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningStrain field around dislocationsEdge dislocations introduce compressive, tensile, and shear lattice strains, screw dislocations introduce shear strain only.Dislocations have strain fields arising from distortions at their cores - strain drops radiallywith distance from the dislocation core6MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningInteractions between dislocationsThe strain fields around dislocations cause them to interact (exert force on each other). When they are in the same plane, they repel if they have the same sign (direction of the Burgers vector) and attract/annihilate if they have opposite signs.7MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningThe number of dislocations in a material is expressed as the dislocation density - the total dislocation length per unit volume or the number of dislocations intersecting a unit area. Dislocation densities can vary from 105cm-2in carefully grown metal crystals to 1012cm-2in heavily deformed metals.Where do dislocations come from ?Most crystalline materials, especially metals, have dislocations in their as-formed state, mainly as a result of stresses (mechanical, thermal...) associated with the forming process.http://zig.onera.fr/lem/DisGallery/3D.htmlComputer simulation of Frank-Read source: Emission of dislocation loops from a dislocation segment pinned at its ends8MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningWhere do dislocations come from ?The number of dislocations increases dramatically during plastic deformation. Dislocations spawn from existing dislocations, grain boundaries and surfaces.http://zig.onera.fr/lem/DisGallery/3D.htmlSimulation of plastic deformation in a fcc single crystal (Cu) of linear dimension 15 micrometers.http://merapi.physik.uni-kl.de/~gerolf/Nanoindentation/Emission of dislocation loops in nanoindentationof copper39MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningSlip SystemsIn single crystals there are preferred planes where dislocations move (slip planes). Within the slip planes there are preferred crystallographic directions for dislocation movement (slip directions). The set of slip planes and directions constitute slip systems. The slip planes and directions are those of highest packing density.Since the distance between atoms is shorter than the average, the distance perpendicular to the plane has to be longer than average. Being relatively far apart, the planes can slip more easily relatively to each other.BCC and FCC crystals have more slip systems as compared to HCP, there are more ways for dislocation to propagate ⇒ FCC and BCC crystals are more ductile than HCP crystals. Remember our discussion of close packed planes in FCC and HCP, Chapter 3.10MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningSlip in single crystals - resolving the applied stress onto the slip systemDislocations move in particular directions on particular planes (the slip system) in response to shear stressesapplied along these planes and directions ⇒ we need to determine how the applied stress is resolved onto the slip systems.λφσ=τcoscosRLet us define the resolved shear stress, τR, (which produces plastic deformation) that result from application of a simple tensile stress, σ.φλ11MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningSlip in single crystals - critical resolved shear stressWhen the resolved shear stress becomes sufficiently large, the crystal will start to yield (dislocations start to move along the most favorably oriented slip system). The onset of yielding corresponds to the yield stress, σy(Chapter 6).The minimum shear stress required to initiate slip is termed the critical resolved shear stress:()MAXyCRSScoscos λφσ=τMaximum value of (cosφ cosλ) corresponds to φ = λ = 45o ⇒ cosφ cosλ = 0.5 ⇒σy= 2τCRSS()MAXCRSSycoscos λφτ=σSlip will occur first in slip systems oriented close to these angles (φ = λ = 45o) with respect to the applied stress 12MSE 2090: Introduction to Materials Science Chapter 7, StrengtheningSlip in a single crystalEach step (shear band) result from the


View Full Document
Download Dislocations and Strengthening Mechanisms
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Dislocations and Strengthening Mechanisms and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Dislocations and Strengthening Mechanisms 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?